Clinical examination of female 129/J mice from an in-house production colony revealed protrusions and tumor-like masses of the vulva in 47 of 106 (44%) animals ranging from 6 weeks to 9 months of age. Cultural, serological, histologic, and electron microscopic examinations were performed in an effort to characterize and determine the etiology of these lesions. Cultural and serological results were negative for bacterial, mycoplasmal, parasitic and viral pathogens. Microscopic examination confirmed these vulva masses to be spontaneous vulvar carcinomas. This was the first report to describe vulvar carcinomas in 129/J mice. A study was conducted to determine the effects of the dietary phytoestrogens daidzein and genistein on the incidence of vulvar carcinomas in 129/J mice. Three natural ingredient diets and two purified diets containing predetermined levels of daidzein and genistein were fed to mice for one year. Mice were monitored for vulva carcinomas at monthly intervals. We concluded that daidzein and genistein were significantly associated with an increase in the incidence of vulvar carcinoma in 129/J mice consuming the AIN-76A modified soy protein diet.